Human beings have built walls for thousands of years. So the proposed Mexican Wall should be no surprise.
The Great Wall of China is the most famous, built in pieces from 600 BC to the twelfth century. Each piece was built by a dynasty, such as the Ming, who wanted to keep out others. The others then breached the wall, killed off that dynasty, started their own dynasty and built more walls. And so on. After several of these cycles, the Wall was really long – and hard to maintain.
With the Great Wall in disrepair, Genghis Khan and his Mongols, mobile horsemen, shrugged and went around the standing sections and conquered the dynasty of the time. Genghis did not build walls, but spent his energies conquering most of the civilized world over 6 decades. So much for national walls. With Genghis and the gang, along with Attila the Hun and his mob, most of the then-civilized parts of Europe and Asia were constantly being killed and robbed.
Europeans, tiring of this cycle, started building walls. Realizing that national walls were not practical, they began walling cities. Every European country, from the 10th through the 16th century, had one or more. It seemed like a good idea. Close the gates, keep out the intruders, and get on with life.
But walled cities were not practical either. You could close the gates – but only if you had a food and water supply. Because the invaders, with nothing else to do, unconcerned about visas, would camp outside. They, of course, had plenty of food and water. So eventually, the gates had to open and the battling began.
Fast forward to the Middle Ages. The nomads went home, to do what nomads do, but warfare never ceased. But now it was nation against nation, their nation attacking your walled city. And now technology was here. Invading armies, not content to wait like nomads, brought along tools. First, there were machines like catapults that could hurl rocks at the walls. Not really efficient, but an army with little else to do could hurl boulders all day for weeks until the walls crumbled. Or, if it was really a big army, with more men than it needed, could attack the walls with ladders and eventually get over to open the gates.
Eventually, using Chinese technology that was better than their walls, armies had gunpowder. Cannons could throw boulders or iron balls from a distance and batter down the wall. Assuming they had brought enough iron balls, it ended badly for the city.
So the smarter Kings and Emperors began to take the wall-building money and build up armies. The era of the Wall was over.
But some nations never learn. After the debacle of WWI, much of which was fought in France, the French decided enough was enough and decided to fall back on traditional methods. They built a wall to keep out the Germans: The Maginot Line. This was a wall to behold. 750 km long, from the Belgian border to Switzerland. A line of forts, concrete gun emplacements, bomb proof shelters, tank traps, and thick walls. More concrete than Las Vegas, and it took 11 years to build.
So when WWII inevitably came along, the French felt warm and snug behind their wall. The Germans, however, did not build walls. They had spent their money on a huge mobile army (see Genghis Khan) with tanks, armored artillery, and troop carriers. Looking at the Maginot Line, the Germans – impressed by its might – simply said: “Scheisse! That’s one big mother of a wall. Let’s just go around it.” And so they hooked a right turn, drove up to Belgium, made a U-turn and invaded France by racing through wall-less Belgium, and were soon in Paris. So much for walls.
(Magician David Copperfield did go through the Great Wall of China in 2003, but I suspect that was just a trick.)
The point is: walls have never worked.